This invention relates to expansible chamber energy conversion machines and more particularly, it concerns a novel apparatus for varying the stroke of a reciprocable piston in such machines with or without substantial variation in the volumetric ratio of the expansible chamber defined in part by the piston.
Expansible chamber energy conversion machines typically include Otto or Diesel cycle internal combustion engines, heat engines in which fuel is burned externally, such as steam engines and Stirling engines, air motors, as well as machines which operate to transform mechanical energy to fluid and/or thermal energy, such as compressors and pumps. In all such machines, operating efficiency is optimized when the work performed in the expansible chamber is correlated with the load demand for converted energy. Where the energy conversion requirements are widely variable therefore, variation in the volumetric capacity of the expansible chamber is significant in the attainment of optimum efficiency under all operating conditions.
It is known that the thermal efficiency of a conventional automotive internal combustion engine, for example, progresses to a near maximum level at about 30% of the rated maximum horsepower of the engine but decreases rapidly with a decrease of power output. Normal power requirements of an automotive engine, however, are most often less than maximum engine capacity with the result that average efficiency over the life of the engine is much lower than optimum. Attempts have been made in the past to permit variation of power capacity in automotive engines by varying the piston stroke or displacement to achieve maximum thermal efficiency under varying power output requirements. Difficulties have been encountered, however, not only in the achievement of a satisfactory mechanical arrangement for varying the length of piston stroke, but also in maintaining the compression ratio of the engine within a range which is acceptable for combustion of a fuel with a given octane rating and from the standpoint of maintaining a constant maximum temperature of the thermal cycle.
Another example of the need for a variable piston stroke in internal combustion engines has been recognized in diesel engines. Diesel engines typically employ relatively high compression ratios which contribute to difficulties both in engine starting and in loss of efficiency at high speed operation. The problems associated with starting an engine having a high compression ratio are manifested by the starting torque required to perform the work necessary to overcome the compression of gas in the combustion chambers of the engine. During high speed operation of diesel engines, typically high compression ratios result in mechanical friction losses of such magnitudes as to detract materially from overall engine efficiency at such speeds.
In engines of the type which convert thermal energy derived from combustion of fuel externally of the expansible working chamber defined in part by piston connected with a rotary shaft, such as the Stirling engine, for example, the provision of a variable piston stroke may be used advantageously to control the output power of the engine. Such engines now employ complex working fluid control systems to this end. Also the B.T.U. output of refrigeration compressors, for example, may be advantageously correlated with load demands by changing the length of the piston stroke in a manner to reduce or increase the fluid displacement for each complete cycle of operation. While the many advantages of varying the stroke of a piston in an expansible chamber energy conversion machine have long been recognized, the difficulties experienced in the attainment of an acceptable mechanical system by which a rotary shaft movement can effectively be transmitted to piston reciprocation, or vice versa, has been a deterent to realization of these advantages.
In a co-pending application for U.S. patent, Ser. No. 706,291, filed July 19, 1976, by the present inventor, various embodiments of internal combustion and Stirling engines are disclosed in which a plurality of pistons are positioned for reciprocation on axes spaced equidistantly and parallel to a central output shaft to which output torque is transmitted by a novel variable speed transmission. Essentially, the transmission includes a pair of axially adjustable, oppositely inclined, conical rolling surface defining members keyed on the output shaft and frictionally engaged at two points of contact by axially adjustable ring-like members carried by a nutating cylindrical member disposed on an axis inclined with respect to output shaft axis. The pistons are connected to the nutating member and synchronized in operation such that the axis of the nutating member develps a bi-conical orbital path about the output shaft. Speed variation is attained by relative axial adjustment of the shaft carried conical rolling surfaces with respect to the ring-like elements on the nutating member to vary the effective diameter of the respective frictionally engaged surfaces. This organization results in an extremely well-balanced, simplified and highly effective arrangement by which the reciprocation of pistons developed by thermal energy may be transmitted to a variable speed, rotary output shaft.